1. Field of the Invention
The present invention relates to an ophthalmic apparatus, and more particularly to an alignment mechanism of the apparatus relative to a patient's eye.
2. Description of Related Art
Referring to the ophthalmic apparatus such as an ophthalmic laser surgery apparatus, a refractometer, a noncontact tonometer, a fundus camera or the like, after the patient's eye fixes a fixation target to fix a position of an eyeball, a desired part of the patient's eye is aligned with the apparatus (such as optical axis of the following optical system: a laser irradiating optical system, a measuring optical system, a photographing optical system or the like) to carry out surgery, measurement, photographing or the like of the patient's eye. Deviation between the patient's eye and the apparatus may occur due to movement of the patient's eye (the eyeball) after alignment. Every time when the deviation occurs, manually-operated alignment is inconvenient. For this reason, currently, a mainly used type of apparatus is the one detects positional relationship between the patient's eye and the apparatus to move the apparatus relative to the patient's eye to perform alignment.
As for the ophthalmic laser treatment apparatus, it is well known that a corneal surgery apparatus which irradiates a corneal surface with an excimer laser beam to ablate a lesion on the corneal surface or to change the corneal curvature to correct refractive errors. For instance, in this kind of the apparatus, movement of the patient's eyeball may occur while the laser irradiating. However, the examiner may not recognize this movement and keeps the laser irradiating, which results in ablating the cornea in a not-predetermined shape. For the prevention of this error, it is suggested to provide the ophthalmic apparatus with a function of moving an irradiating optical axis of the laser irradiating optical system to perform tracking of the patient's eye. In this kind of tracking mechanism, an anterior part of the eye is consistently photographed with a CCD photographing element while the laser irradiating. Then, in the case of detecting that the pupil center position or the like are not positioned within a predetermined allowable range relative to the irradiating optical axis or the like as a standard position, the tracking of the irradiating optical axis is performed in a manner to make the pupil center position or the like coincide with the standard position based on the photographing image.
As for the above-mentioned CCD photographing element for position detecting, a CCD photographing element for interlaced scanning which alternately outputs image data (image signals) of an odd field and of an even field is generally used because such a CCD photographing element is economical. However, this CCD photographing element requires about 33 ms to obtain a screenful of image data. For this reason, in case that the movement of the patient's eye is fast, it results in decreasing accuracy of tracking and operation.
In addition, it is suggested that the apparatus which projects target luminous flux on the patient's cornea and photographs a target image (bright spot) formed on the cornea with the CCD photographing element to perform tracking based on the results. However, in case that the patient's eye moves fast, which results in decreasing accuracy of tracking and surgery.